This invention relates to a method for the production of an aluminum nitride sintered body and, more particularly, relates to an unsintered aluminum nitride body and a method for forming such body followed by sintering so as to produce an aluminum nitride sintered body having a high density and satisfactory thermal conductivity by sintering at a temperature lower than that in conventional techniques.
Aluminum nitride has been of interest recently for electronic packaging applications because of its high thermal conductivity, thermal expansion matching with silicon, low dielectric constant (8.5) and high electrical resistivity.
Production of an aluminum nitride sintered body having excellent thermal conductivity requires forming of aluminum nitride powder followed by sintering to achieve a dense body.
Since aluminum nitride by itself is difficult to sinter under atmospheric pressure, pressureless sintering of aluminum nitride has conventionally been carried out with the aid of additive sintering aids.
For example, Takeshima et al. Japanese Kokai J04-154,670 discloses dense, sintered aluminum nitride bodies which have been achieved by the use of alumina and calcia additions.
Most of the additives proposed previously are highly refractory materials, and remain in the solid state during the early stage of sintering. During the course of sintering, some of them eventually form a liquid phase by reacting with any aluminum oxide present, either by intentional addition or with the aluminum oxide impurity in aluminum nitride. The liquid phase thus formed has been reported to aid densification of aluminum nitride. However, because of the refractory nature of these additives, the temperature required for sintering aluminum nitride has been exceptionally high (1800-2000 degrees Centigrade) compared to the sintering temperature of 1500-1600 degrees Centigrade for alumina.
Others have proposed lower melting additives such as boria in addition to calcia and alumina. Nakano et al. Japanese Kokai J02-275,769 discloses adding additions of aluminum, calcia and boria to aluminum nitride powder, followed by sintering at 1400-2000 degrees Centigrade. However, to achieve a fully dense body having a thermal conductivity of 192 W/m-K, the compositions were sintered at 1800 degrees Centigrade for 4 hours. Sawamura, et al., Japanese Kokai J62-176,961 discloses additions or mixtures of alumina, calcia and boria (as well as others) to aluminum nitride to achieve a sintered body. In one example, a mixture of 7 weight percent 3Al.sub.2 O.sub.3 .multidot.5CaO and 1 weight percent B.sub.2 O.sub.3 was added to aluminum nitride and sintered for 2 hours. The resulting thermal conductivity was 70 W/m-K. Boria, by itself, however, melts at about 450 degrees Centigrade which presents difficulties in electronic packaging applications. For example, it is necessary to remove substantially all residual carbon from substrates that are used in electronic applications. The low melting boria hinders this so-called binder burnoff process.
Finally, others have proposed the addition of sintering aids in the form of vitreous materials. Saito et al. Japanese Kokai J03-218,977 discloses the addition of 0.1-10 weight percent of a glass powder sintering aid to the aluminum nitride powder prior to sintering. The glass powder consists of 0-38 mole % alumina, 30-80 mole % boria and 20-56 mole % calcia. In weight percent, it is 0-28 weight % alumina, 27-77 weight % boria and 23-64 weight % calcia. The aluminum nitride body is sintered at a temperature greater than 1650 degrees Centigrade which is undesirably high. The resulting aluminum nitride samples have a maximum thermal conductivity of 110 W/m-K which, while better than alumina, is considerably less than pure aluminum nitride. Further, the majority of samples, however, had a thermal conductivity of 100 W/m-K or less.
Accordingly, the present inventors have proposed adding a calcia-alumina-boria glass to aluminum nitride powder in a way, and in an amount, sufficient to obtain a dense, highly thermally conductive body.
It is thus a purpose of the present invention to produce an aluminum nitride body that is dense and highly thermally conductive.
It is another purpose of the present invention to produce an aluminum nitride body by a sintering process at a lower sintering temperature than has heretofore been possible which will allow the production of the aluminum nitride body at a reduced cost.
These and other purposes of the present invention will become more apparent after referring to the following detailed description of the invention.